Compositions of polyvinylpyridine and iodine

ABSTRACT

Compositions of and methods for the preparation of polyvinylpyridine and iodine (PVP.nI 2 ) as cathode materials for electrochemical cells.

BACKGROUND OF INVENTION

This is a continuation of application Ser. No. 6,003, filed Jan. 24,1979 abandoned.

Batteries, such as those of the lithium/iodine type for example,sometimes referred to as solid state cells, make use of a cathodematerial comprised of iodine and an iodine containing charge transfercompound. Charge transfer compounds are sometimes referred to ascomplexes and sometimes as donor-acceptor compounds. The iodine in sucha cathode reacts electrochemically with the lithium anode to provide avoltage output. This reaction causes a lithium iodide electrolyte toform in situ between the anode and cathode. The charge transfer donortypically used is a polyvinylpyridine, (PVP) such aspoly-2-vinylpyridine (P2VP) or poly-4-vinylpyridine (P4-VP). Additionalamounts of free iodine ie., excess iodine which is not combined with thecharge transfer donor are usually included as part of the cathodematerial to provide an iodine "reservoir" for the battery to draw onduring discharge. The additional iodine increases the useful life of thebattery.

Unfortunately, the polyvinylpyridine-iodine cathode compositions,previously used such as P2VP and/or P4VP based cathodes have exhibited aconductivity which degrades severely with the addition of relativelylarge amounts of iodine to the complex. Such amounts of iodine arehereinafter referred to as "excess iodine". Consequently, thisdegradation in the conductivity of the composition has effectivelylimited the amount of iodine which could heretofore be included in suchcathodes without degrading the conductivity of the material below auseful level. As a result, the polyvinylpyridine cathode materials,which have been previously available, have not had as high an energydensity as is theoretically possible from an iodine based system. Thisfact is particularly important for such applications as in implantablemedical devices, ie., heart pacemakers for example.

It is a purpose of this invention to provide polyvinylpyridine-iodinecathodes, P2VP and/or P4VP based, having markedly higher conductivitythan has been heretofore possible. As a result, the preparation ofuseful cathode materials having a much higher iodine content, such asmaterials having a final mole ratio of 20:1 or greater overall iodine toP2VP and/or P4VP donor, is made possible. Consequently, higher energydensities and longer useful life are provided by the improved cathodematerials prepared according to this invention.

Herein, "mole ratio" is defined in terms of the number of moles ofiodine (I₂) to the number of gram formula weights of vinylpyridine inthe initial polymer-donor mixture. For example, a mixture whichinitially contains 508 grams of I₂ and 10.5 grams of PVP would have amole ratio (n) of 20:1 and will be designated as PVP.20I₂. "Molefraction" is defined in an analogous manner as n/n+1, according to theabove nomenclature.

For descriptive purposes herein, the term "complex" refers to any singlephase iodine and donor mixture. The term "cathode material" refers to amaterial composed of a "complex" and may include excess iodine, whichmay be present as a solid phase.

SUMMARY OF THE INVENTION

The invention is directed to cathode materials and provides methods ofpreparing cathode materials from P2VP, P4VP or mixtures thereof withvarying amounts of iodine, At least a part of the preparation must takeplace at temperatures in excess of about 225° C. in a sealed containerin order to obtain the improved conductivities, which are characteristicof this invention.

In one form of the invention, following preparation of a highlyconductive cathode material at elevated temperature, excess iodine isthen added. The iodine may be added at room temperature and in amountsto provide a final mole ratio of about 20:1 or higher if desired. Forexample, a final mole ratio of 40:1 may be prepared in this manner.Smaller amounts may also be used.

In another form of the invention, a one-step technique may be used toprepare a high conductivity cathode material directly by initiallyadding the desired final amount of iodine. In this embodiment thedesired relative final amounts of P2VP and P4VP donor and iodine aresimply mixed together and heated to a temperature in excess of about225° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing conductivity plotted versus reaction time forsamples of P2VP based cathode materials prepared at various reactiontemperatures as indicated;

FIG. 2 is a graph showing conductivity plotted versus both "molefraction I₂ " and "mole ratio I₂ to P2VP" for samples of cathodematerials prepared at various reaction temperatures, as indicated, witha four hour reaction time;

FIG. 3 is a graph showing the conductivity versus reciprocal temperatureof cathode materials made from P2VP and P4VP donor materials (PVP.3.3I₂)prepared at various temperatures, as indicated, with a four hourreaction time;

FIG. 4 is a graph showing the voltage output versus capacity (expressedin amp-hours) of a battery having a P2VP based cathode material preparedaccording to the invention, and,

FIG. 5 is a graph showing conductivity plotted versus reactionpreparation heating temperature for the P2VP.3.3I₂ samples of cathodematerials illustrated in FIG. 1, reaction time at temperature isindicated for each.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is best illustrated by reference to the FIG. 1 graph whichshows the conductivity (measured at 37° C.) of samples of P2VP.3.3I₂(mole ratio) mixture heated at various temperatures plotted versus timeof heating to form a cathode material. Heating is accomplished in asealed container. It can be seen that heating at 175° C. will produce aconductivity no higher than about 4×10⁻³ reciprocal ohm-cm no matter howlong the heating takes place. On the other hand, heating such a mixtureat about 225° C. produces a complex having a conductivity (at 37° C.) ofabout 7×10⁻³ reciprocal ohm-cm. Heating at even higher temperaturesproduces cathode materials having even higher conductivities. It is alsoshown in FIG. 1 that less heating time is required to obtain improvedconductivity as the heating temperature is increased above about 225° C.for preparing cathode materials. Thus, a predetermined minimum time ofheating or reaction time is required depending on the heating orreaction temperature used in the preparation.

The P2VP.3.3I₂ materials produced by heating the components together inexcess of about 225° C. are in and of themselves desirable cathodematerials because of their high conductivities.

Other mole ratios of iodine mixed with P2VP donor behave in a similarmanner ie., heating above about 225° C. provides higher conductivity, asis illustrated by the graph of FIG. 2. However, the mole ratio should beat least 1:1. In FIG. 2, the conductivity (measured at 37° C.) ofcathode materials of various mole ratios [n in (P2VP.nI₂)] are plottedfor various preparation or reaction temperatures. It can been seen that,for any selected mole ratio, 3.3 for example, the conductivity thereofis higher as one proceeds to higher preparation temperatures. It ispreferred that the cathode material have a conductivity higher thanabout 1.5×10⁻³ reciprocal ohm-cm and that the mole ratio be at least12:1. This can be readily accomplished at temperatures of about 225° C.or higher.

P4VP donor material behaves similarly to P2VP donor material. This isillustrated in FIG. 3 which shows the conductivities of 3.3:1 mole ratiocathode materials made up from P2VP and P4VP donor materials with fourhours reaction time at 150° C. and 320° C. In this Figure, theconductivities are plotted versus reciprocal temperature to show theirrelative activation energies. The upper 320° C. and upper 150° C. linesrepresenting P2VP, the lower lines representing P4VP. The P2VP and P4VPcomplexes, prepared at 150° C. have similar slopes but differentabsolute conductivities. This indicates their conductivity mechanism islikely the same and the conductivity difference probably arises fromthese polymers inherent properties. However, when the cathode materialsare prepared at 320° C. both the slopes and absolute conductivities arevery similar. Thus, this invention also provides conductivityenhancement in P4VP based cathode materials which are comparable to P2VPbased counterparts. All of the data suggest P2VP and P4VP behaveanalogously with regard to this invention.

Mixtures of P2VP and P4VP donor material may also be used. For example,a mixture of equal parts P2VP and P4VP was used to prepare a 3.3:1 moleratio cathode material. The material prepared at 320° C. for four hourshad a conductivity of 1.15×10⁻² reciprocal, ohm-cm.

High conductivity in the cathode material is necessary because, asalready pointed out, high additional amounts of iodine added to thematerial degrades its conductivity. By providing a high initialconductivity in the material, larger amounts of iodine can be addedthereto without degrading the final conductivity of the resultantmaterial to a low undesirable value. With higher ratios, one can expectto maintain a conductivity of about 10⁻³ reciprocal ohm-cm or betterwith the techniques of this invention while also obtaining a muchimproved deliverable energy density than has been heretofore possible.As can be seen in the graph of FIG. 4, a P2VP.20I₂ (mole ratio) cathodematerial prepared according to the invention and tested in a batteryhaving a lithium anode provided about 90% of stoichiometric capacity toa 2.0 Volt cut-off. The cathode material was prepared by heating amixture of P2VP.3.3I₂ to 300° C. for about four hours in a sealed glasscontainer. Upon cooling, excess iodine was added to the material toprovide a final mole ratio of P2VP.20I₂.

Cells made according to the invention at 20:1 mole ratio from P4VP basedmaterials behave similar to the P2VP based material prepared accordingto the invention. For example, a P4VP cell had an initial open circuitvoltage (OCV) of 2.74 volts as compared with 2.78 volts for a P2VP cell.At 10 k ohm load the P4VP cell had a voltage of 2.65 volts while acomparable P2VP cell had a voltage of 2.60 volts. The lack ofsubstantial differences between these cells indicates that the P4VPbased cathodes will enjoy performance enhancement similar to that forthe P2VP cells when prepared according to the invention.

The ability to include large quantities of excess iodine with morehighly conductive materials, formed with the polyvinylpyridine polymerand iodine, is an important feature of this invention from the energydensity standpoint as already indicated.

There are two ways to incorporate the excess iodine with the material.In the first and presently most preferred form, the polyvinylpyridinepolymer component is mixed with a relatively low amount of iodine, suchas 3.3 mole ratio or less, ie., enough to merely form a single phasecathode material with little or no excess iodine. As previouslydescribed, the mixture is heated in excess of about 225° C. for apredetermined period of time, dependent upon the temperature selected,to form material of improved conductivity. For example, as shown in FIG.1, heating a 3.3 mole ratio sample at about 225° C.-320° C. for aboutone hour produces improved material. An 8 hour reaction time appears tobe adequate to assure complete reaction. However, longer times may beused. An additional amount of iodine may then be added, at anyconvenient temperature ie., room temperature or elevated, to provide anydesired excess amount in the final cathode material resulting from thispreparation.

More specifically, in accordance with this first form of the invention,a highly conductive material is prepared from P2VP, P4VP or mixturesthereof ie., P2VP.nI₂ and/or P4VP.nI₂ where n (indicative of mole ratio)may range from about 1.0 to 3.3. A preferred value for n is 3.3 in thecase of P2VP and iodine. Such a preferred composition prepared inaccordance with this invention will exhibit a conductivity on the orderof 4×10⁻³ reciprocal ohm-cm or higher. Optionally, additional iodine maythen be added.

The high conductivity obtained in the cathode materials prepared by thefirst method of the invention is illustrated by referring again toFIG. 1. The Figure also demonstrates the practical parameters of timeand temperature involved in the method of the invention. Several samplesof P2VP.3.3I₂ cathode materials were prepared in sealed containers atvarious temperatures and times ranging over 150° C. to 320° C. and fromone hour to twenty-four hours. The 3.3:1 mole ratio materials used inpreparing this Figure are illustrative. As already pointed out, variousother ratios are possible as desired.

As can be seen from FIG. 1, samples heated at 250° C. for at least about8 hours, upon cooling and measuring the conductivity thereof at 37° C.,exhibit a conductivity on the order of 10⁻² reciprocal ohm-cm orgreater. On the other hand, the time for heating at 320° C. can beshortened to on the order of an hour or so to attain or exceed a 37° C.conductivity of the order of 10⁻² reciprocal ohm-cm. The 37° C.temperature at which conductivity is measured for samples discussedherein was selected arbitrarily. Other temperatures may be used formeasuring the conductivity of these materials so long as any selectedtemperature is used consistently for purposes of comparison betweensamples.

Heating temperatures in excess of 350° C. show no substantial increasedbenefit as to heating time in attaining the high conductivity levels ofthis invention. The graph of FIG. 5 illustrates this aspect using thesame samples as were used for FIG. 1. In FIG. 5 the conductivity of thesamples, in reciprocal ohm-cm at 37° C., is plotted against temperatureto show that there is an effective maximum temperature to which all ofthe samples converge. The maximum temperature is on the order of about325°-350° C. From a practical standpoint, heating at temperatures inexcess of this level appears to be of no benefit.

As seen in FIG. 3, P4VP based compounds can be prepared over the sametemperature range as those of P2VP.

Upon cooling to room temperature, following the above preparation, thehigh conductivity P2VP, P4VP or mixed P2VP/P4VP cathode material will bemore or less fluid depending on the amount of iodine included thereinand the temperature to which it was heated.

In order to prepare a high energy density cathode material it is onlynecessary to mix additional iodine with the above described materials.Preferably, the iodine is ground into a convenient powder form for thispurpose. Elevated temperatures may be used but are not necessary. Theamount of iodine may be selected to provide any desired final mole ratiorelative to the P2VP and/or P4VP organic constituents. In the case ofbatteries for implantable medical devices, it is preferred that thefinal mole ratio be of at least the order of 12:1.

For example, using a P2VP.3.3I₂ material prepared according to theinvention, having a conductivity in excess of 10⁻² reciprocal ohm-cm andbeing fluid in form, powdered iodine was added thereto in sufficientquantity to provide a material having a final mole ratio of about 20:1.The resultant cathode material was of a wet sand-like consistency anddark appearance. It was pressed to a density of about 4.7 g/cc and usedin a battery.

In the second form of the invention, highly conductive cathode materialsare prepared ranging over various iodine mole ratios by simply includingthe desired final amount of iodine in the heating container with thepolyvinylpyridine polymer, sealing the container and heating it to atemperature greater than about 225° C. for a predetermined timedependent on the selected temperature.

The table shows the conductivity of several samples of 20:1 mole ratiocathode material prepared according to one or the other of the two formsof the invention.

The first four cathodes were prepared by the first form or the two-stepmethod using initial mole ratios between 1:1 and 6.2:1 and diluting withadditional iodine to provide the final 20:1 mole ratio.

The table also shows a sample prepared according to the second form ofthe method of the invention ie., sufficient iodine being added initiallyto result in a cathode material having a final 20:1 mole ratio. As canbe seen, all samples had conductivities between 1.2 and 2.1×10⁻³reciprocal ohm-cm.

                  TABLE                                                           ______________________________________                                                              Conductivity of                                         Reaction Temperature Composition*                                                                   Final P2VP . 20I.sub.2                                  (Mole Ratio I.sub.2 :P2VP)                                                                          at 37° C. (ohm-cm).sup.-1                        ______________________________________                                          1:1                 1.25 × 10.sup.-3                                  2.1:1                 1.39 × 10.sup.-3                                  3.3:1                 1.75 × 10.sup.-3                                  6.2:1                 2.05 × 10.sup.-3                                   20:1                 1.56 × 10.sup.-3                                  ______________________________________                                         *Reaction time 4 hours at 320° C.                                 

Examples of PVP which may be used with this invention are shown below.PVP from other sources will also be satisfactory for use with theinvention.

P2VP may be synthesized as follows:

Benzoyl peroxide (2.0 grams) is dissolved in freshly distilled2-vinylpyridine (200 grams). Water (400 ml) is added and the mixture ispurged with nitrogen for 1 hour. With continued purging, the mixture isheated at 85° C. with stirring and kept at that temperature for twohours. The organic phase will thicken and develop a brown color duringthis time. The mixture is cooled; the aqueous phase is discarded and theorganic phase is dried overnight at 60° C. in a vacuum oven. The residueis ground into fine granules and dried to a constant weight at 60° C. inthe vacuum oven. Yield 162 gm (81%) poly-2-vinylpyridine. This productcan be expected to provide the following results upon analysis by gelpermeation chromotography:

Weight - average molecular weight--555,000

Number - average molecular weight--199,000

P2VP is manufactured by the Ionac Chemical Co., Birmingham, Alabama:typical weight--average molecular weight--301,000; typicalnumber--average molecular weight--128,000.

P4VP may be synthesized as follows:

Freshly distilled 4-vinylpyridine is purged with nitrogen for one hour.The 4-vinylpyridine is heated with stirring under a continuing nitrogenpurge to 160° C. and maintained at that temperature for 90 minutes. Thecontents of the reactor will darken and thicken during this time untilagitation becomes very difficult to maintain. The reaction product isthen poured warm into a container for storage and tightly sealed.

The product can be expected to provide the following results uponanalysis by gel permeation chromatography:

% volatiles (probably monomer)--34%

Average molecular weight--6000 (includes monomer).

P4VP may be obtained commercially from Polysciences, Inc., identified as#0112.

Having described the invention, the embodiments thereof in which anexclusive property right is claimed are defined in the followingclaims:
 1. A composition comprising an initial polyvinylpyridine polymercomponent, selected from the group consisting of P2VP, P4VP and mixturesthereof, and iodine in a predetermined mole ratio of from about 20:1 toabout 40:1, the composition having a conductivity measured at 37° C. ofat least about 1.5×10⁻³ reciprocal ohm-cm and being formed in a sealedcontainer by mixing the polymer component and at least a portion of theiodine together and by heating the polymer component before or duringmixing, or by heating the resultant mixture, to a temperature in excessof about 225° C., and thereafter adding the remaining balance, if any,of the iodine to make-up the predetermined mole ratio.
 2. A compositioncomprising an initial polyvinylpyridine polymer component, selected fromthe group consisting of P2VP, P4VP and mixtures thereof, and iodine in apredetermined mole ratio of at least about 20:1, the composition havinga conductivity measured at 37° C. of at least about 1.5×10⁻³ reciprocalohm-cm and being formed in a sealed container by mixing the polymercomponent and at least a portion of the iodine together and by heatingthe polymer component before or during mixing, or by heating theresultant mixture, to a temperature in excess of about 225° C. andthereafter adding the remaining balance, if any, of the iodine tomake-up the predetermined mole ratio.
 3. The composition of claim 1wherein the mole ratio is from about 20:1 up to about 40:1 and theconductivity at 37° C. is at least about 3×10⁻³ reciprocal ohm-cm.